1) Field of the Invention
This invention relates to a high-refractive index plastic optical material, and especially to a high-refractive index plastic optical material low in dispersion.
2) Description of the Related Art
In optical instruments, a variety of inorganic glass lenses have heretofore been used. In recent years, plastic lenses have however begun being used widely together with the inorganic glass lenses owing to their good lightweight property, processability, stability, dyeability, high-volume production capability and reducibility in cost.
Among various properties required for optical lenses, it is extremely important for them to have a high refractive index and be low dispersion. According to a lens composed of an optical material high in refractive index, it is possible to make a lens system, which occupies an important position in optical instruments such as microscopes, cameras and telescopes and spectacle lenses, compact, and hence to reduce its weight and control aberration due to spherical surface and/or the like to a small extent. On the other hand, the low dispersion of the optical material is extremely important in that the chromatic aberration of the lens becomes small.
As with materials for inorganic glass lenses, however, materials for plastic lenses also show a tendency to be high dispersion if they are high in refractive index, but to be low dispersion if they are low in refractive index on the other hand. For example, there is mentioned a diethylene glycol bisallyl carbonate resin designated "CR-39" as a plastic material for spectacle lenses, which is most widely used at present. Although this resin has an Abbe's number as high as 60 (namely, low in dispersion), its refractive index is as extremely low as 1.50 (as measured at 20.degree. C.; the same shall apply to the following). Besides, polymethyl methacrylate which is being used in a partial field as a material for lenses is also high in Abbe's number, but has a refractive index as low as 1.49.
On the other hand, polystyrene, which is credited with being relatively high in refractive index and low in dispersion, has a refractive index of 1.59 and an Abbe's number of 30.4. Similarly, polycarbonate has a refractive index of 1.59 and an Abbe's number of 29.5. However, other physical properties of these synthetic resins are far from satisfactory as materials for lenses. For example, polystyrene is low in surface hardness and moreover lacking in solvent resistance. Besides, polycarbonate is low in surface hardness and moreover lacking in impact resistance. Further, polynaphthyl methacrylate, which has a refractive index of 1.64, and polyvinylnaphthalene, which has a refractive index of 1.68, have an Abbe's number as considerably low as 24 and 20, respectively. Neither of them is hence said to be a good optical material.
As described above, there has heretofore been known no plastic optical material having excellent optical properties that the refractive index is high and the dispersion is low, and moreover possessing good physical properties.